Clutches are used generally to control the transmission of torque between rotating machine elements. Positive clutches, when engaged, lock the elements together to rotate as one. Such clutches may be "self-energizing" to the extent that the force of engagement increases as the torque between the shafts increases.
It is known to produce electrical, self-energizing, positive clutches using a helical spring attached at one end to one rotating machine element, and electromagnetically attracted to the second rotating machine element by an electromagnetic coil when the clutch is to be activated. Twisting of the helical spring caused by relative motion of the two rotating machine elements causes the spring to tighten about an annular wedge of frictional material to press it against opposed faces of the two machine elements bringing them into frictional linking. The helical spring and the wedge shape of the frictional material define a mechanical advantage that may allow a relatively low force of attraction between the electromagnetic and the helical band to nevertheless provide a high torque coupling of the two machine elements. One example of such a clutch is found in U.S. Pat. No. 3,149,706, filed Sep. 22, 1961 and hereby incorporated by reference.
Often it is desirable, when the clutch is disengaged, to lock the driven machine element in place to prevent free-wheeling. For this purpose, an electric brake may be attached to the driven machine element. Circuitry must be provided to energize the brake when the electric clutch is de-energized. The use of an electric brake increases the cost and size of the total drive mechanism.